a. Field of the Invention
The invention relates to an optical system for a projection display and, more particularly, to an optical system for a projection display capable of providing high image contrast and a wide viewing angle.
b. Description of the Related Art
A projection display typically consists of an illumination system and a projection system. The illumination system incorporates a light path switching device that consists of a plurality of relatively small elements each being used to switch light path individually. After being modulated by the switching elements, light beams emitted from a light source are projected on a projection surface through the projection system.
A digital micromirror device (DMD) manufactured by Texas Instruments (TI), as an example of a light path switching device, is composed of thousands of micromirrors. The DMD panel's micromirrors are mounted on tiny hinges that enable them to tilt either toward the light source (ON mode) or away from it (OFF mode), thus creating a light or dark pixel on the projection surface.
FIG. 1 is a schematic view showing a conventional optical system 100 for a projection display. Referring to FIG. 1, the tiltable micromirrors on a digital micromirror device 102 may either direct the incoming light l onto a projection lens 104 along path 108 under the “On mode” or direct it away from the projection lens 104 along path 110 under the “Off mode”, thereby creating a light or dark pixel on the projection surface.
A total internal reflection (TIR) prism set 106, composed of two prisms 106a and 106b adhered to each other with an air gap 112 interposed therebetween, is disposed in a light path between the digital micromirror device 102 and the projection lens 104. The TIR prism set 106, inside which total internal reflection occurs at the boundary between the prism 106b and the air gap 112, guides the incoming light l to the digital micromirror device 102 along the light path shown in FIG. 1.
However, through such design, since the tilting range of the micromirror is limited, the light path of the incoming light l between the digital micromirror device 102 and the projection lens 104 under the On mode is almost the same as that under the Off mode; hence, an edge portion of the spread-out incoming light l enters the projection lens 104 under the Off mode to result in a deterioration in the image contrast. Though this problem may be solved by increasing the distance between the projection lens 104 and the TIR prism set 106 to prevent stray light from entering the projection lens under the Off mode, the back focal length, however, is increased accordingly, and thus it is difficult to design a projection lens with a wide viewing angle.
FIGS. 2A and 2B are schematic views showing another optical system 200 for a projection display. The TIR prism set 206 of the optical system 200 includes three prisms, and air gaps 208 and 210 are formed between each two adjacent prisms. Under the On mode as shown in FIG. 2A, the incoming light l enters the digital micromirror device 202 due to the total internal reflection occurring at the boundary between the air gap 208 and the prism. Then, the light l reflected by the micromirror on the digital micromirror device 202 passes through the TIR prism set 206 and enters a projection lens 204 along a non-reflected optical axis. On the other hand, as shown in FIG. 2B, under the Off mode the light reflected by the micromirror on the digital micromirror device 202 is reflected outside the optical system 200 due to the total internal reflection occurring at the boundary between the air gap 210 and the prism. Such TIR prism set 206 may render the light paths under the On mode and the Off mode more distinguishable to prevent stray light from entering the projection lens. However, the width W along the non-reflected optical axis of the assembled TIR prism set 206 becomes larger and the back focal length is increased, thus it is also difficult to design a projection lens with a wide viewing angle.